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Post by haydukej on Jan 10, 2015 18:00:55 GMT -5
I won a dead SCXD chassis on ebay awhile back for $50. And from what I saw on the pictures it looked like three resistors were shot and there was some black staining around one of the 6v6 tube inputs (that must have been fun to see). So I took the gamble, figuring I was somewhat handy with an iron in hopes that I could salvage this amp for somewhat cheap. Somehow, I knew I wouldn't get off that easy though. In short, I need help troubleshooting it if anyone would be so kind. The only other discussions I've seen on here was Cynical1's effect loop efforts for it. Here's where I'm at so far, which isn't much. SCXD Schematic
One can download the service manual from here if interested/needed. - As I stated the obvious issues were 3 blackened resistors (specifically: R22, R24, and R79), which I replaced last night.
- I also had to buy a power switch and fuse holder for it as those were missing
- The amp was also sans tubes, so I ordered new ones as well.
After those three updates, I got to through the switch and bupkis. No red jewel, no sounds and no smoke (which was the only thing good). With my limited knowledge, I would first attempt to check the trannys out to make sure they're not fried, but I'm not sure how to go about that. I'm open to any other plans as well that may be more logical. Thanks
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Post by haydukej on Jan 21, 2015 23:54:14 GMT -5
Well, leave a man alone enough and he'll either figure it out or die trying. After some google reviewing of how to safely test PTs, I've confirmed that this one is shot on the primary side. All secondaries seem to have the appropriate resistance, but the primary was wide open. Further confirmed by applying power and then not seeing any come through on the secondaries.
In short, if anyone runs across a broken down SXCD with a good PT in it, I'd be much obliged for a heads up.
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Post by sumgai on Jan 22, 2015 12:30:47 GMT -5
'duke, Sorry man, you caught me in the hospital, and I wasn't worth much for a few days afterwards. As you've seen, the power transformer primary was blown wide open. The clue here, at least for me, was the power switch gone AWOL. That's a clue that the prior owner had gotten at least as far as "nothing coming out, so probably nothing going in... the fuse is good, so it must be - the power switch!". I would've said to do exactly as you did, take resistance readings first. Beats thumping one's head against the bench, as one realizes that the shotgun approach to troubleshooting doesn't work too well. HTH sumgai
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Post by haydukej on Jan 22, 2015 13:17:43 GMT -5
SG, no worries. I should have been more proactive in exercising my searchfu prior to posting, given the simple procedure. Hope the trip to the hospital wasn't anything too exciting and the road to recovery is a quick one.
I'm wondering how feasible it would be to power everything with multiple trans, since the original is apparently so specialized. I saw the tubedoctor (UK) will make them on special request for about 104 euros, not to mention shipping and 6 to 8 weeks lead time. I imagine they might stick me with a 240v version though. Plus I'd like to get it fixed for a bit cheaper than that if possible.
Does anyone happen to see a way I could check to make sure the DSP side is still functioning prior to the transformer replacement?
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Post by sumgai on Jan 22, 2015 20:28:04 GMT -5
'duke, There's no "easy" way to check the DSP functions without voltage... they kinda go hand in hand. However, as you've noted, replacement transformers are few and far between. (Just in case you didn't already know, the Deluxe VM (vintage modified) uses the same power transformer.) That said, I think I'd either haunt eBay, or else start collecting various other transformers for supplying my voltage needs. Again, in the event that you didn't already know this/hadn't figured it out yet, you need to supply these voltages: 285vAC (for the plates of the 6L6's); 16vAC (for some of the digital stuff); 9vAC (for the remainder of the digital stuff); 6.3vAC (for the tube filaments/heaters) Individual transformers may be bulky and expensive, but for the most part, they'll be more reliable. Good luck! sumgai
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Post by haydukej on Jan 23, 2015 11:08:29 GMT -5
SG, I did see a mention (by Billm) that the Deluxe VM PT could be swapped, and not to question his or you electronic skills by any means, but on the respective schematics, I see that the SCXD PT has a center tap on one of the secondaries while the Deluxe VM / Bandmaster VM schematic does not. That seems like a critical detail to me, but I've been wrong before.
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Post by sumgai on Jan 23, 2015 15:30:42 GMT -5
Heydy, I should've looked more closely myself, but for a different reason... I now retract what I said earlier about the DVM or BVM transformers being good replacements - I don't like the fact that the B+ voltage is about 60vDC higher for these models, compared to what's going on in the SCXD. 6V6 tubes won't like that higher voltage, it exceeds their max rating by more than 15%, according to most tube manuals. (Although.... in order to achieve higher wattage ratings, most power tubes are overdriven with a bit more B+ than called for. (Like perhaps 5 to 7%,) This explains why we see "XYZ" wattage ratings, but we almost never see at what percentage of distortion is that rating achieved. For the most part, the ratings we see for the vast majority of guitar/bass amps is set at 5 (five) percent!!!!!. IOW, what we call a clean-sounding amp is, in reality, dirty as all get-out. But we like it, and that's the bottom line. So for those who insist on spouting that "specs tell the story", no, you're wrong. One's ear tells the story, and that's the whole story, period. Sorry if I trounced on any toes, but there it is, in black and white, for all the world to see.) 'duke, if you want to try contacting Fender directly (or through an authorized retailer/service center), the exact part number you need is: 007459200 That's their number for the 120vAC version of the Power Transformer, according to the service manual. Availability and pricing subject to Fender's whims, of course. HTH sumgai
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